livecheck: Clarify --extract-plist behavior

From the description of the `--extract-plist` option, it would seem
that the `ExtractPlist` strategy is only enabled when the option is
used. Instead, livecheck automatically enables the strategy if the
command is run on only one cask. This rewords descriptions of the
option to clarify the behavior.

Library/Homebrew/dev-cmd/livecheck.rb

@@ -37,7 +37,7 @@ module Homebrew
         switch "--cask", "--casks",
                description: "Only check casks."
         switch "--extract-plist",
-               description: "Include casks using the ExtractPlist livecheck strategy."
+               description: "Enable checking multiple casks with ExtractPlist strategy."
 
         conflicts "--debug", "--json"
         conflicts "--tap=", "--eval-all", "--installed"

Library/Homebrew/livecheck/livecheck.rb

@@ -218,7 +218,7 @@ module Homebrew
         )
       end
 
-      # If only one formula/cask is being checked, we enable extract-plist
+      # Allow ExtractPlist strategy if only one formula/cask is being checked.
       extract_plist = true if formulae_and_casks_total == 1
 
       formulae_checked = formulae_and_casks_to_check.map.with_index do |formula_or_cask, i|